Long-term effects of anthropogenic habitat disturbance on a lizard assemblage inhabiting coastal dunes in Argentina

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1 1653 Long-term effects of anthropogenic habitat disturbance on a lizard assemblage inhabiting coastal dunes in Argentina Laura E. Vega, Patricio J. Bellagamba, and Lee A. Fitzgerald Abstract: We studied abundance and habitat use in two species of Liolaemus (Squamata: Tropiduridae) at a coastal dune site in eastern Argentina before and 7 years after a road was built at the site. Before disturbance, lizards exhibited similar abundances and a wide segregation in microhabitat use. Liolaemus multimaculatus used flat dunes scarcely covered by Spartina ciliata, while Liolaemus gracilis used the grass Panicum racemosum as cover. After disturbance, the mean number of L. multimaculatus detected by month was significantly less than that observed in the predisturbance period, owing to a drastic reduction in S. ciliata microhabitat patches. The mean number of L. gracilis was similar to that seen during the first period. These differences were clearly linked to habitat loss at the site. We concluded that human impact on the habitat structure of foredunes induced changes in the structure of the lizard assemblage, including shifts in the relative abundance of species and the proportional use of their preferred microhabitats. Résumé : Nous avons étudié l abondance et l utilisation de l habitat chez deux espèces de Liolaemus (Squamata : Tropiduridae) sur le site d une dune côtière de l est de l Argentine, avant et 7 ans après la construction d une route à cet endroit. Avant la perturbation, les lézards des deux espèces étaient tout aussi abondants et il y avait ségrégation des deux dans différents microhabitats. Liolaemus multimaculatus utilisait les dunes plates à peine couvertes de Spartina ciliata, alors que Liolaemus gracilis utilisait les zones couvertes de plantes herbacées Panicum racemosum. Après la perturbation, le nombre moyen de L. multimaculatus repérés chaque mois était significativement moins élevé qu avant la perturbation à cause de la réduction considérable des microhabitats à S. ciliata. Le nombre moyen de L. gracilis a peu changé après la perturbation. Ces différences sont directement attribuables à la perte d habitat sur le site. Nous concluons que l impact de la perturbation d origine humaine sur la structure de l habitat des avant-dunes a entraîné des modifications des associations de lézards, notamment des changements dans l abondance relative des espèces et dans l utilisation proportionnelle des habitats préférés. [Traduit par la Rédaction] 1660 Introduction Vega et al. Human activities along the northern Atlantic coast of Argentina have led to an increase in coastal erosion (Isla and Villar 1992). The major causes of anthropogenic erosion are urban development (Bertola et al. 1999), inadequate beach management (Isla et al. 1994), and sand mining (Farenga et al. 1992; Isla 1992). The pressure of urban development, specifically the building of roads and houses within 150 m of the high-tide mark, leads to the abrupt eradication of Received November 11, Accepted April 28, L.E. Vega. Facultad de Ciencias Exactas y Naturales, Departamento de Biología, Universidad Nacional de Mar del Plata, Funes 3250, 7600 Mar del Plata, Argentina. P.J. Bellagamba. Subsecretaría de Medio Ambiente, Municipalidad de General Pueyrredón, La Rioja 1650, 7600 Mar del Plata, Argentina. L.A. Fitzgerald. 1 Department of Wildlife and Fisheries Sciences, Texas Cooperative Wildlife Collection, Texas A&M University, College Station, TX , USA. 1 Author to whom all correspondence should be addressed ( lfitzgerald@tamu.edu). 2 Isla, F.I., and Villar, M.C Ambiente costero : Pacto Ecológico. Informe inédito del convenio Universidad Nacional Mar del Plata Cámara de Diputados de la Provincia de Buenos Aires, Argentina, unpublished report. Can. J. Zool. 78: (2000) dunes or sets up barriers that alter the dynamic sand exchange. Inevitably, the dunes slowly waste away (Echeverría 1987; F.I. Isla and M.C. Villar 2 ). Urban pressure, along with the cultivation of exotic plants to stabilize the dunes and the establishment of tourist facilities on the coast of Buenos Aires Province, Argentina, have led to a continuous process of habitat loss and fragmentation. This pattern has been repeated in many other coastal areas of the world (Sorensen et al. 1992). The ecological consequences of coastal dune degradation for small vertebrates, particularly ecological interactions among arenicolous lizards, are generally unknown. Two studies have reported effects of coastal habitat alteration on the herpetofauna in South America. Gudynas (1989) described the assemblage of reptiles and amphibians in an altered site, and Rocha and Bergallo (1992) found that a gradual reduction in beach vegetation over 10 years was accompanied by a decline in the population of Liolaemus lutzae in southeastern Brazil. Dune lizard assemblages in Buenos Aires Province contain up to four species, one of which is endemic to coastal dunes. Previous work (Vega 1994) showed that these species differ in their preference for structural microhabitat characteristics and in exhibiting a high degree of spatial segregation in sympatry. Studies on the effects of habitat disturbance on reptiles in Argentina are lacking, as are studies in which a before-and-

2 1654 Can. J. Zool. Vol. 78, 2000 after design was used to evaluate the effects of coastal dune disturbance on vertebrate communities. To properly document effects of anthropogenic disturbance on the coastal dune fauna, before-and-after data on ecology and habitat are required. Pre- and post-disturbance data constitute powerful evidence concerning how organisms may be threatened by changes in habitat (Schlesinger and Shine 1994). In the present study, we used before-and-after comparisons to evaluate the effects of anthropogenic habitat disturbance, specifically the construction of a road through a coastal dune system, on two Liolaemus species. Each sampling period lasted 15 months, sampling was done at the same place, and identical methods were used. We were therefore able to examine directly the effects of disturbance 2 years before, 7 years after construction of the road. Our goal was to determine if and how habitat disturbance and alteration affected lizard abundance and microhabitat use and point out the implications of our findings for conservation. Materials and methods Study organisms Liolaemus multimaculatus is a relatively small tropidurid lizard (70 mm snout vent length (SVL)) endemic to coastal sand dune habitats of Buenos Aires and Rio Negro provinces in Argentina (Cei 1993). It belongs to the wiegmanni group of Liolaeminae (Etheridge 1995) and occurs only on substrates of aeolian sands (Etheridge 1993). It is insectivorous and oviparous and reproduces during the austral spring and early summer, October December. Activity is depressed during winter months, April July (Vega 1997). Liolaemus gracilis is a small (55 mm SVL), slender lizard belonging to the chiliensis group of Liolaeminae (Etheridge 1995). It has a larger distribution than L. multimaculatus and occurs in central, southwestern, and eastern Argentina and along the Atlantic coast from Chubut to Buenos Aires provinces (Cei 1993). Liolaemus gracilis occurs on a variety of substrate types including sand (Gallardo 1977). Little is known about the ecology of L. gracilis. Study area The study site was an area of coastal dunes at Rocas Negras, Mar del Sud, Buenos Aires Province (38 21 S, W). Mean annual temperature was 13.5 C with a high monthly mean of 21 C in January and low monthly mean of 7.5 C in July (Servicio Meteorológico Nacional 1988, 1998). Annual mean rainfall was >830 mm and there is no marked wet or dry season. The area consisted of 5 ha of sand and gravel beach dunes over a low ravine. The landscape became flatter away from the sea, forming a small hill bordering pampas grassland. Habitats at the study area formed a vegetation gradient from coast to inland along the beach, foredunes, and back dunes. Foredune vegetation was dominated by Spartina ciliata ( espartillo ), Panicum racemosum ( tupe ), and Poa barrosiana. Backdune vegetation was characterized by Adesmia incana, Poa lanuginosa, and Lagurus ovatus (Cabrera 1940). The spatial arrangement of these vegetational habitats was a mosaic of five relatively discrete patches of different plant associations in foredunes and a more homogeneous grassy area in the back dunes (Fig. 1A). It was important to describe the configuration of this habitat mosaic and the patchiness of the study area in order to test for effects of habitat disturbance. Sampling We visited the study site monthly from October 1984 to December 1985 (15 monthly visits). During each visit, we censused lizards visually by working our way systematically through the foredunes ( m) and searching all habitats for lizards (Campbell and Christman 1982). Each census began at 11:00, corresponding to peak daily activity of lizards (L.E. Vega and P.J. Bellagamba, personal observation) and lasted approximately 1 h. The study area was thoroughly canvased during the censuses. Species and ageclass were recorded for each lizard observed. Subadults were <45mm SVL and immature in overall appearance. Before each census we recorded air temperature 1 m above ground. To quantify microhabitat use by individual lizards, we quantified dominant plant species and vegetation cover using Webb s ocular estimating classes (de Vos and Mosby 1971) in a 1-m 2 plot around the exact location where a lizard was seen. Rock cover was quantified using the same method. A road crossing the study site was built in Seven years post construction, from October 1994 to December 1995 (15 monthly visits), we returned to the same site to quantify the effects of road construction on the lizard assemblage. The same census and habitat-measurement protocols were followed at the same site. Because the numbers of lizards seen during censuses in were positively correlated with air temperature on the census day (L. multimaculatus: r = 0.74, n = 15, p < ; L. gracilis: r = 0.56, n = 15, p < 0.02), surveys in were matched as closely as possible for weather conditions. Analyses To test the null hypothesis that abundances of the two lizard species did not differ between and , we used Wilcoxon s paired signed-ranks tests to compare the actual numbers of lizards seen during each census between the two time periods. We used contingency tables to test the null hypothesis that there was no difference between observed and expected frequencies of substrate use between lizard species (Zar 1984). To test the null hypothesis that there was no difference in microhabitat use by each species, we quantified the amount of each habitat available to the lizards, then calculated expected frequencies of lizards in each microhabitat according to the surface area of each microhabitat during each time period. Results Habitat alteration Removal of sand and vegetation during road construction caused a drastic reduction in the original vegetation in foredunes (Fig. 1B. Use and maintenance of the road prevented accumulation of sand at this location. Progressive erosion during subsequent years exposed a slime sand clay soil in erosive banks. Seven years after construction, the original homogeneous sandy habitat, m 2 in area and covered by S. ciliata, was reduced to two thin strips beside the road that totaled 1500 m 2. Although scattered clumps of S. ciliata remained, bare sand predominated. Consequently, this patch was reduced to 10% of its original area (Fig. 2). The patch of P. racemosum vegetation in hummock dunes was fragmented and finally reduced to <50% of its former area (from 1500 to 700 m 2 ). In contrast, the patches of P. barrosiana and Poa sp. in the foredunes and the rocky patch at the beach did not change noticeably in area. Neither visible damage nor decreased vegetation cover was detected in the back dunes (Fig. 2). Lizard abundance The number of L. multimaculatus detected during the 15 monthly surveys in (mean = 0.93, SD = 1.48, n = 15) was much lower than the number observed in (mean = 7.3, SD = 12.2, n = 15) and this difference

3 Color profile: Disabled Composite Default screen Vega et al Fig. 1. Photographs of the study area showing the reduction in amount of vegetative cover and changes in the mosaic of microhabitat patches in the beach dune system at Rocas Negras, Buenos Aires Province, Argentina, in , before road construction (A), and in , 7 years after road construction (B). was statistically significant (Wilcoxon s paired signed-ranks test, Z = 2.81, p < 0.005, n = 15; Fig. 3). The mean number of L. gracilis seen by month in was 6.9 (SD = 6.5, n = 15) and was not statistically distinguishable from the number seen per month in (mean = 6.7, SD = 8.9, n = 15) (Wilcoxon s paired signed-ranks test, Z = 0.13, p > 0.90, n = 15; Fig. 3). Numbers of both species peaked during the austral spring and early summer (September December), corresponding to recruitment of hatchlings. Age structure (the ratio of adults to juveniles) also varied with time of year and recruitment of juveniles. The age structure was 4:1 for L. multimaculatus in December in both 1985 and The largest number of L. gracilis in a census in 1985 was 33 in October. The age structure was 1:3. The largest number seen during the post- construction period was 22 in May 1995, and the age structure was also 1:3. Habitat use During , lizards used habitats in patches of rock along the beach and in vegetation communities in foredunes. Each species was statistically significantly associated with particular habitats, taking into account the surface area of each habitat available (Table 1). More L. multimaculatus were found than expected in S. ciliata and less in P. barrosiana and P. racemosum (total χ2 = , df = 3, p < 0.005; Table 1), while L. gracilis was almost exclusively found in P. racemosum (total χ2 = , df = 3, p < ; Table 1). Following substantial changes in proportions of habitat available after disturbance of the dunes, the J:\cjz\cjz78\cjz-09\Z vp Thursday, August 24, :38:29 AM

4 1656 Can. J. Zool. Vol. 78, 2000 Fig. 2. Maps of the study area in and , depicting alteration of the landscape and microhabitat mosaic of the beach dune system before and after road construction. Dramatic changes in the lizard community between and were associated with notable reductions in the area occupied by Spartina ciliata and Panicum racemosum. same pattern of habitat use was observed in Numbers of L. multimaculatus were drastically reduced, from 110 in to 14 in This species remained associated with S. ciliata despite the reduction of this habitat from 59 to 9% of the available area (total χ 2 = 7.64, df = 3, p < ; Table 1). It is possible that L. multimaculatus experienced a subtle change in the rate of habitat use during the postdisturbance period. In , the proportion of the area consisting of rocks increased from 25 to 75%, and 8 of the 14 L. multimaculatus were found there. This number still was lower than expected, however, based on habitat availability. Liolaemus gracilis remained strictly associated with P. racemosum following disturbance (total χ 2 = , df = 3, p < ; Table 1). Habitat segregation was also very evident between the two lizard species during each sampling period, reflecting the distinct habitat associations of each species (total χ 2 for either year > , df = 3, p < ; Table 1). The amount of vegetation cover at microhabitat sites occupied by individual lizards differed between species during both time periods ( : total χ 2 = , df = 2, p < 0.000; : total χ 2 = 26.85, df = 2, p < , respectively; Table 2). More L. multimaculatus were sighted than expected at sites with <33% vegetation cover, while more L. gracilis were seen at sites with >33% vegetation cover. More L. multimaculatus were found than expected in where substrate cover was >33% (total χ 2 = 21.76, df=2,p < ; Table 2), possibly because these lizards increased their use of rocks during the postdisturbance period. In contrast, the pattern of cover used by L. gracilis did not vary between periods (total χ 2 = 0.74, df = 2, p = 0.69; Table 2). It appeared the L. gracilis were concentrated in

5 Vega et al Fig. 3. The number of each lizard species per transect during and Table 1. Proportions and area of habitat availability and proportions and numbers of individuals using each microhabitat during and Habitat availability L. multimaculatus L. gracilis Proportion Area (m 2 ) Proportion n Proportion n Rocks Spartina ciliata Panicum racemosum Poa barrosiana Total Rocks S. ciliata P. racemosum P. barrosiana Total remaining clumps of P. racemosum where suitable cover was available. In summary, habitat use by these lizards can be described in terms of dune topography, type of plant association, and mean vegetation cover of sand substrate. The preferred habitat of L. multimaculatus was S. ciliata clumps in relatively low dunes. Individuals were detected close to the base of these grasses or running over bare sand. Liolaemus multimaculatus were found hiding under rocks on the beach, though less frequently than in S. ciliata. The preferred habitat of L. gracilis was P. racemosum in rugged dunes. Liolaemus gracilis were detected moving under vegetation on dune slopes, and they avoided open spaces. Discussion Before the road was built, populations of both lizard species exhibited similar abundances and wide segregation in habitat use. Each species mostly used a particular part of the habitat that differed in amount of vegetative cover and in plant composition. Liolaemus multimaculatus used open spaces in flat dunes scarcely covered by S. ciliata. Their cryptic

6 1658 Can. J. Zool. Vol. 78, 2000 Table 2. Proportions of two lizard species using different substrate vegetation cover classes at microhabitat sites during and Substrate cover (%) L. multimaculatus L. gracilis < (90) 0.06 (6) (20) 0.89 (93) > (0) 0.05 (5) < (6) 0.04 (4) (6) 0.89 (95) > (2) 0.07 (7) Note: Numbers in parentheses are sample sizes. coloration, specialized morphology, and behavior (Cei et al. 1975; Gallardo 1977; Halloy et al. 1998) are concordant with a lizard species highly specialized for arenicolous environments. Liolaemus gracilis is a generalist, occurring in a variety of habitat types throughout its range (Gallardo 1977; Cei 1993). At our study site, L. gracilis were found almost exclusively under clumps of P. racemosum and avoided open spaces. The habitat associations we observed were consistent with the pattern of habitat use documented for these species in a regional study along 350 km of coastal dunes (Vega and Bellagamba 1992). Seven years after disturbance of dunes, the lizard populations and their habitat at Rocas Negras were clearly different. Differences included drastic changes in relative abundance, most notably the near disappearance of L. multimaculatus. We documented a shift in the proportional use of preferred microhabitat by L. multimaculatus. The 14 L. multimaculatus detected in were found in the few remaining places with scattered clumps of S. ciliata and in the rocky area surrounding the S. ciliata patch. In the postdisturbance sampling, this lizard species showed a tendency to use open space near rocks more than S. ciliata. While the results of the tests were statistically significant, the pattern was difficult to interpret because so few L. multimaculatus were seen in Rocks were more available than S. ciliata in , presumably because of progressive erosion at the site (Isla et al. 1997). It is possible the shift in habitat use occurred in relation to the changing availability of microhabitats. Relative frequencies of L. multimaculatus using sand among patches of Poa spp. were also higher during the postdisturbance period than before, but small numbers of lizards seen near Poa spp. in both sampling periods precluded the use of statistical tests (Table 1). While a few L. multimaculatus were seen in Poa spp., it appeared that this habitat was not preferred by L. multimaculatus. Importantly, the shift in habitat use by L. multimaculatus apparently did not compensate for the loss of preferred Spartina habitat in terms of recruitment. The 90% reduction in S. ciliata habitat was accompanied by an 87% reduction in the number of L. multimaculatus. No obvious pattern of annual rainfall explained the decrease in the L. multimaculatus population. Annual rainfall in the region during was not significantly different from that in ( : mean = 802 mm, SD = 180; : mean = 701 mm, SD = 95; t 0.05,6 = 2.45, p < 0.32) (Servicio Meteorológico Nacional 1988, 1998). Rainfall patterns apparently had no negative effect on L. gracilis at our site, nor on a L. multimaculatus population 60 km south of the study area (Vega 1999a). The most perceptible change at the site was the altered landscape. The road reduced and destroyed patches of vegetation and caused soil erosion. Vegetation is an essential element of most lizards habitat in xeric biotopes such as sand dunes. It provides thermal refugia and protection from predators, may provide nesting sites, and also provides habitat for arthropod prey consumed by lizards (Rocha 1988, 1989, 1995, 1996; Vega 1999b). Rocha and Bergallo (1992), concluded that a progressive reduction of beach vegetation over the last 10 years has reduced populations of the endemic lizard, L. lutzae, in Barra de Tijuca Reserve in Brazil. Considering the apparent dependency of L. multimaculatus on S. ciliata, it seems clear that the drastic reduction of this grass at our study site was the cause of the decrease in numbers of these lizards. In spite of the 50% reduction of the P. racemosum area, the L. gracilis population was as abundant as 7 years before and exhibited a similar age structure. In contrast to the situation with L. multimaculatus, the decrease in area did not drive L. gracilis into alternative habitats. Instead, the frequency of use of remaining P. racemosum by L. gracilis increased. Several mechanisms could account for the constant number of L. gracilis, but it is impossible to determine from our data which of these may have been operating. The densities of L. gracilis that were measured could have reflected temporary peaks and lows in a fluctuating population (Fitzgerald 1994). Additionally, the lizards were concentrated in small remnants of P. racemosum in , which may have facilitated our finding them. Alternatively, the L. gracilis population at this site may be limited by factors other than space, such as food availability or predation risk, that could allow relatively constant numbers to persist irrespective of density. Regardless of mechanisms governing the population dynamics of L. gracilis, it is clear that this species was tightly linked to P. racemosum cover. Vertebrates may respond to disturbance by undergoing changes in life-history strategies, behavior, ecology, and physiology. Similar types of disturbance may produce different effects depending on the size of impacted area and the type of patch considered (Karr and Freemark 1985). For instance, if the intensity or persistence of disturbance differed among similar habitat patches, organisms using different patches might respond in different ways (Wiens 1985). This scenario is relevant in our case, since the effects of road construction varied among patches of S. ciliata and P. racemosum. The lizard species we studied showed a high degree of habitat segregation and each responded differently to habitat alteration. Liolaemus multimaculatus is a habitat specialist that depends on relatively open sandy substrates among patches of S. ciliata. In accordance with the prediction that specialists will be more sensitive to disturbance, L. multimaculatus showed a drastic decline. Based on our findings we raise the following scenario for the Rocas Negras disturbance. When the road was built through the patch of S. ciliata, habitat used by L. multimaculatus was destroyed, causing the loss of many individual lizards almost immediately. The maintenance of the road and the increase in tourist activities and erosion prevented the recovery of

7 Vega et al S. ciliata, which in turn resulted in poor population recovery and limited colonization of L. multimaculatus. In the case of L. gracilis, the road probably contributed indirectly to modification of its habitat by increasing erosion and gradually reducing the P. racemosum habitat to one-half its original area. Hence, the habitat used by both species was reduced because of the road, though by different means. Habitat reduction had a direct impact on population size, density, and area occupied by each species. While we have focused on each species independently, anthropogenic change in habitat structure is also expected to result in modification of lizard assemblages and alter the dynamics of species interactions. Vitt et al. (1998) documented how changes in relative availability of microhabitats and microclimates associated with single-tree harvesting resulted in conditions favoring heliothermic lizard species over shadedwelling species. The structure of the lizard assemblage at our study site was altered through changes in relative abundance, population density, and use of space by both species. Other than diet, we did not directly measure lizard species interactions in the dunes. It does seem likely, however, that community-level patterns at our study site were altered by anthropogenic disturbance. We have demonstrated that conservation of lizard species in sand dunes along the coast of Argentina relies on the maintenance of native patches of S. ciliata and P. racemosum in foredunes. Although road construction in sand dunes causes easily perceived changes in habitat, our study provides insight into mechanisms of biodiversity loss in altered coastal sand dunes. Acknowledgements Many thanks are extended to J. Chani for help in the field and to V. Morales for help with illustrations. L. Vitt and G. Dayton made helpful comments. 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